Surface Coating Techniques
A vast variety of materials can be deposited as thin films or coatings. This invention particularly focuses on ceramic coatings. Ceramic coatings are often used for properties like, hardness, friction, corrosion resistance, and biocompatibility.
The most established techniques for depositing ceramic of coatings are CVD (Chemical Vapor Deposition), PVD (Physical Vapor Deposition), electrolytic deposition, and thermal spray deposition. Furthermore, there are numerous subgroups within each deposition technique.
CVD is a high temperature process (typically 800° C.–1000° C.), wherein a chemical reaction occurring between the surface of a substrate and a gas that is flooded over the surface, generates a surface film on the substrate. The technique is mainly used for deposition of metal carbides, -nitrides or -oxides upon temperature resistant substrates, such as hard metal. The thickness of the deposited films may be in the range from nanometers to micrometers.
PVD is based on physical processes, most often plasma techniques, and may be used at lower temperatures than CVD, typically 300° C.–500° C. at the substrate surface. Contrary to CVD, PVD processes are line of sight processes, which imply that it is not possible to deposit films around corners, inside tubes, etc. PVD may be used for deposition of pure metals and a large number of chemical compounds. PVD methods are commonly used for deposition on temperature sensitive substrates, such as steels, aluminum and even plastic materials. The coating thicknesses are of the same order of magnitude as for CVD processes.
Thermal spray deposition includes techniques based on gas flame, electric arc and gas plasma, all of which involve extremely high temperatures. The melting zone may reach temperatures in the order of 10 000° C. This set requirements on the temperature properties of both substrate and coating materials. Thermal spraying may be used for deposition of a number of metallic and ceramic materials. In general, the deposited films are thicker than for PVD and CVD, in the range of 100 micrometers to a few millimeters.
A disadvantage with existing techniques for deposition of ceramic films is the elevated temperatures required in the process. Therefore, the most preferred of the above methods is often PVD, which may be used for deposition around 300° C. Another disadvantage is that said methods require advanced deposition equipment, especially CVD and PVD, for which gas-tight vacuum-arrangements are needed.
The main disadvantage with thermal spray deposition is the temperatures of the melting zone. Also the cooling rate of the deposited material is extremely high. Cooling from typically 10 000° C. to room temperature in a few microseconds, implies that possibilities to control the micro-structure of the coating are very limited. Phase-composition, chemical composition, porosity and surface structure cannot be accurately regulated.
Biocompatible Materials
As for bioceramics, hydroxyapatites, or other calcium phosphates, are of particular interest. Hydroxyapatite is osseo-compatible, since bone tissue regenerates excellently against this ceramic. The material seems to be capable of forming a direct bond with natural bone. One reason for this may be that human bone tissue is composed of about ⅔ of hydroxyapatite.
As pure bulk material hydroxyapatites and other calcium phosphates have poor mechanical properties. Hydroxyapatite is therefore often used as a coating material on metal substrates or as an additive in a stronger matrix (see WO/11979). Polymer based bone cements with hydroxy apatite fillers is an established product. However, all techniques involving elevated temperatures tend to alter the microstructure of the hydroxyapatite, e.g. that the hydration water in the hydroxyapatites leaves the structure.
Orthopedic components with hydroxyapatite based coatings deposited with various thermal spraying techniques, form a relatively large group of implants. Attempts have also been made to produce bio (or osseo-) compatible coatings of coral-like materials of calcium carbonates. However, the limited mechanical properties of these materials set limitations to their use.
Calcium Aluminates
Another bioceramic is calcium aluminate, and its medical applications is described e.g. in S. F. Hulbert, F. A. Young, R. S. Mathews, J. J. Klawitter, C. D. Talbert. It is shown that tissues (bone, muscular, subcutaneous fat) to a large extent do not react when put in contact with pure calcium aluminate, i.e. no irritation, inflammations, or toxic reactions occur.
SE-463 493 discloses a chemically bound ceramic material comprising aluminates and silicates. The material is achieved through a production technique involving pre-compaction of the ceramic body. In addition, the ceramic material may comprise an inert phase of e.g. hydroxyapatite or oxides of titanium, zirconium, zinc and aluminium.
Calcium aluminate has been explored as a tooth filling material, e.g. the product Doxadent® produced by Doxa Certex AB, see e.g. PCT/SE99/01729, “Sätt att framställa en kemiskt bunden keramisk produkt, samt produkt”, 29, Sep. 1999.